A conventional hydraulic clutch control for motor vehicles has a master cylinder which is connected to a reservoir filled with hydraulic fluid. The master cylinder has a cylinder housing in whose housing bore a piston is held longitudinally displaceably. The master cylinder piston in the housing bore of the cylinder housing defines a pressure chamber and by means of a clutch pedal or an electric motor drive can be mechanically pressurized by an activating force. The pressure chamber of the master cylinder is hydraulically connected via a pressure line with a pressure chamber of a slave cylinder so that the pressure generated in the pressure chamber of the master cylinder by pressing the clutch pedal or electric motor movement of the master cylinder piston can be transferred via the fluid column in the pressure line to the pressure chamber of the slave cylinder. The slave cylinder also has a cylinder housing with a housing bore in which is held longitudinally displaceably a piston which delimits the pressure chamber of the slave cylinder and can thus be pressurized with the pressure generated in the master cylinder. Allocated to the slave cylinder piston is a piston rod which acts on a clutch lever which in turn is actively connected with the clutch release bearing. As a result the clutch release bearing can, via the slave cylinder piston, be pressurized with an activation force in order, via a release mechanism, to separate the clutch pressure plate from the clutch carrier plate and hence the engine from the gearbox of the motor vehicle.
In particular in connection with hydraulic clutch controls which are supplied to the assembly line of the motor vehicle manufacturer as preassembled units of master cylinder, reservoir, pressure line and slave cylinder, prefilled with hydraulic fluid, the prior art (e.g. U.S. Pat. No. 4,665,802, U.S. Pat. No. 4,766,804, GB 2 241 298 A) has already proposed temporarily tying the piston rod of the slave cylinder in a predetermined stroke position in relation to the cylinder housing of the slave cylinder in order to avoid overfilling of the hydraulic clutch control, for its transport and to simplify installation of the unit in the motor vehicle.
Thus the GB 2 241 298 A reference discloses a slave cylinder with a cylinder housing having a housing bore, a piston held longitudinally displaceably in the housing bore and a piston rod allocated to the piston, in which slave cylinder is provided a fixing element which before first operation of the slave cylinder is detachably connected with the piston rod and fixes this in a predetermined stroke i.e. axial position in relation to the cylinder housing, and which furthermore is designed on first operation of the slave cylinder to release the piston rod in relation to the cylinder housing. In this prior art the fixing element has a ring flange section which is inserted in a stepped end section of the housing bore of the cylinder housing at its open end and fixed there by means of a locking ring. From the ring flange section of the fixing element, several thin fixing strips extend radially inwards to the end of the piston rod facing away from the piston where they transform as one piece into the piston rod head in order to tie the piston rod in relation to the cylinder housing in a predetermined stroke position before first operation of the slave cylinder. When the slave cylinder is activated for the first time, the fixing strips on the ring flange section tear off and release the piston rod.
Although in relation to previous solutions (e.g. U.S. Pat. No. 4,665,802, U.S. Pat. No. 4,766,804) in which the fixing element had a separate retaining cap placed on the piston rod head which was detachably clamped by means of fixing strips in relation to the outer periphery of the cylinder housing in order to hold the piston rod in a predetermined stroke position in relation to the cylinder housing, this prior art has the advantage that assembly of the slave cylinder is considerably easier because of the one-piece design of the fixing element and piston rod, the generic-state of the art requires further improvement in particular in functional respects. In the generic prior art it is not possible to attach to the head of the piston rod facing away from the piston a slip cup or dust protection cap swivellable in relation to the piston rod, as is known for example from the prior art according to DE 195 16 392 A1 or DE 100 38 012 A1. Also in operation of the slave cylinder problems can arise in that the fixing strips formed on the piston rod head and torn away from the ring flange section of the fixing element hinder the function of the slave cylinder as they obstruct any necessary stroke-dependent angular movement of the piston rod in relation to the center axis of the cylinder housing.
What is needed is a hydraulic cylinder designed as simply as possible, in particular a slave cylinder for a hydraulic clutch control for motor vehicles, in which before first operation of the hydraulic cylinder the piston rod is tied in a predetermined stroke position in relation to the cylinder housing and is released on first operation of the hydraulic cylinder without the problems outlined above in the discussion of the prior art.